U.S. Department of Agriculture: Wheat Trade - http://www.ers.usda.gov/Briefing/Wheat/trade.htm
Exports are crucial for the U.S. wheat industry, with exports averaging nearly half of total use during the 1990s. During the 1990s, wheat and wheat flour accounted for over 9 percent of all U.S. agricultural exports by value, indicating the importance of wheat exports to the U.S. economy.

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USDA-Marketing and Trade - http://www.usda.gov/wps/portal/!ut/p/_s.7_0_A/7_0_1OB?navtype=SU&navid=MARKETING_TRADE
The USDA has helpful tools to understand the marketing and trade of the agriculture business both domestic and international.